Fracture and energy evolution of rock specimens with a circular hole under multilevel cyclic loading

Abstract

Underground structures gradually face increasing multilevel cyclic loading as construction progresses. In this paper, the fracture and energy evolution of rock specimens with a circular hole under multilevel cyclic loading were investigated using an experimental-theoretical approach. A series of multilevel cyclic loading tests were conducted on two types of rock (sandstone and granite) using the MTS 815 rock mechanics test system. Digital image correlation (DIC) and acoustic emission (AE) techniques were employed to monitor the fracture behavior of the rock with deformation and acoustic emission data recorded respectively. The total energy density, elastic energy density, and dissipated energy density of specimens are obtained by theoretical calculation. The results indicate that, despite the similarity in cracking sequence and fracture evolution of rock specimens with a circular hole between multilevel cyclic loading and uniaxial compression tests, there are differences in the AE activity under these two loading conditions. Additionally, the cracks around the hole under multilevel cyclic loading display a distinct “open-close-reopen” behavior. The energy density of rock specimens with a circular hole grows nonlinearly with stress. The elastic and dissipative energy density exhibits a linear relationship with the total energy density before rock failure. The fitting value of the elastic and dissipative energy density can be regarded as the energy storage coefficient and the energy dissipation coefficient, which could be further used to quantify the energy storage and dissipation capacities of rock. Under multilevel cyclic loading, the propagation of remote fractures and their coalescence with the shear fractures is the primary resource of AE energy, with only a few supplies given by micro-fissure closure and primary fractures. The dissipation energy in the rock structure can be described by the growth of AE energy. The findings indicate that there are distinct stages of energy release during the fracture evolution of cracks around the circular hole. These stages can be predicted by changes in AE energy or macroscopic energy, revealing the evolution stages of cracks and the modes of failure.